The present invention relates to coated cemented carbide cutting tools with improved properties obtained by a heat treatment of the cemented carbide before the application of a wear resistant coating. More particulary, the invention is in particular related to a body coated with diamond. However, the invention can also be used for other wear resistant coatings, such as any CVD- or PVD wear resistant coatings.
The adhesion of wear resistant coatings on cemented carbide is due to several mechanisms of which mechanical adhesion constitutes one part, i.e. a rough surface grips mechanically into the applied coating. Several approaches have been made in order to achieve good mechanical adhesion see EP-A-500 253. For the case of diamond layers, the achievement of adhesion is particularly difficult since the presence of binder metal deteriorates the adhesion. A common method for achieving good diamond layer adhesion on cemented carbide is to etch away the binder metal from a surface region, see EP-A-0 500 253. This, however, increases the risk of bad adhesion due to loosely bound carbide grains. Other methods involve carburising sintering of an eta phase containing body (U.S. Pat. No. 4,843,039) so as to deplete a surface region of binder phase. The treatment gives a relatively brittle surface zone together with a likewise brittle core which is less than adequate for certain toughness demanding cutting applications.
Another method for preparing a cemented carbide for diamond coating is described in WO 95/15258 according to which a rough surface is obtained through a heat treatment in N2 containing atmosphere. The surfaces treated in this way are free of binder phase. However, during a heat treatment as described in WO 95/15258 grain growth occurs not only at the surface but also within the body generally resulting in formation of large WC-grains in the structure.
U.S. Pat. No. 4,830,886 describes a method to avoid the presence of the eta phase layer formed during a CVD deposition of a TiCN layer by carburizing the eta phase layer through the TiCN-layer after the CVD process.
EP-A-759 480 describes a cemented carbide body containing triangular WC-platelets. The platelets are oriented with their triangular surfaces parallel to the surface of the body. The toughness of the body is said to be improved.
U.S. Pat. No. 5,100,703 describes a diamond-coated sintered body, comprising a film of diamond and/or diamond-like carbon deposited on the surface of a sintered body of a hard phase composed mainly of a mixture of W2C and WC and inevitable impurities.
It has now surprisingly been found that within a surface zone of a cemented carbide body, a controlled change in WC grain orientation, size and shape can be obtained by an initial decarburization or decarburization+heat treatment in neutral gas atmosphere or vacuum through which an eta phase containing surface zone is obtained and then a complete recarburization of said zone in order to completely transform the eta phase therein to WC+Co. (Eta phase is a common name for low carbon content containing carbides usually comprising Wxe2x80x94Coxe2x80x94C in proportions (M6C or M12C, M=W and Co and/or Fe and/or Ni) such as M12C=Co6W6C and M6C=Co3W3C, W4Co2C). The shape of the WC grains is triangular platelets. The platelet containing region can be combined with different binder phase gradient zones. The adhesion of a subsequently deposited wear resistant coating to this body is found to be excellent.
According to one aspect, the present invention provides a body of cemented carbide comprising 4-12 wt-% binder phase coated with at least one wear resistant layer, the body comprises a surface zone with a thickness up to 100 xcexcm, the zone comprises essentially triangular WC platelets having a texture coefficient ratio  greater than 1.75.
According to another aspect, the present invention provides a method for producing diamond coated cemented carbide body with a surface zone in the cemented carbide being different from the rest of the carbide body, the method comprising: decarburizing the body to form an eta phase containing surface zone 3-100 xcexcm thick and comprising  greater than 60% by volume of at least partially interconnected eta phase and recarburizing the body whereby the eta phase is completely transformed into WC+Co.